Transmission system



Oct. 7, 1947. R. H. HERRICK 2,428,520

TRANSMISSION SYSF'EM Fiied May 1, 1943 g Sheets-Sheet 1' .OUT AMP.CHANNEL (I24 ENABLING a DISABLING UNIT 7 I "0 BALANCING CIRCUIT ENABLING8 DISABLING UNIT,

FIG. I

Q65 l64l IN AMP INVENTOR. ROSWELL H. HERRICK M 1M7, M m

ATTORNEYS LINE I00 Oct. 7, 1947. HERRICK I 2,428,520

TRANSMIS S ION SYSTEM Filed May l, 1943 2 Sheets-Sheet 2 OUT AMP.CHANNEL ENABLING 8| DISABLING UNIT BALANCING cmcun ENABLING 8| DISABLINGUNIT III FIG.- 2

LINE 200 AMP. CHANNEL INVENTOR. ROSWELL H. HERRICK ATTORNEYS PatentedOct. 7, 1 947 UNITED STATES PATENT omce Roswell H. Herrick;

Automatic Electric ration of Delaware Oak Park, Ill., assignor toLaboratories, Inc, 21/601110:

Application May 1 1943, Serial No. 485,269

'12-Claims. l

The present invention relates generally to improvements in signalcurrent transmission system of the type in which signalcontrolledswitching circuits are provided for partially or completelyblocking under certain conditions. certain of the signal channelsincluded therein and, more particularly, to improvements in telephonesubstation circuits or the type disclosed in United States LettersPatent N0. 2,282,405, granted May 12, 1942, upon the application ofRoswell H. Herrick, for Transmission'systems.

In the patent referred to there is disclosed an improved telephonesubstation apparatus of the character wherein the channels arepreferably only partially blocked when not in use, and Where-, in thecontrol of the channel blocking circuit is arranged to utilize a hotcathode controlled gas tube, whereby eifective channel blocking andunblocking operations are obtained Without any sub-. stantial speechclipping.

In such an arrangement there is provided a telephone substationconnected to an associated line by a hybrid or anti-sidetone impedance.network and a'first channel'is provided for trans mitting signalcurrents ol the gas tubesto the as.- sociated channel-so that apredetermined potential is impressed on the control electrode of thetube :to render it conductive when signal current traverses itsassociated channel.

Each of the control circuits is provided with circuit meansoperative tolower the potential difrent arresting means are disclosed. In one ofthese embodiments a sloW-to-release relay is provided in the loadoroutput circuit of the-rectifying means, which relay functionsmomentarily to interrupt the anode circuit of the associated gas control:tube when the input signal voltage derived from the associated channel,and impressed upon the input circuit of the rectifying means, isremoved. In the other embodiment there is Q Y p o ded a source ofalternating current which is utilized as the anode current source foreach of two control tubes. With the latter arrangement, when the controlelectrode of one of the tubes is biased to a starting potential, thetube functions as a half-Wave rectifier so that direct voltages aredeveloped in the output circuit thereof which are utilized to performthe desired char-1 nel control functions.

While such arrangements have proven to be entirely satisfactory it maybe desirable to provide certain other characteristics and advantages notcontained therein. For example, it may be desirableto utilize in placeof a hot cathode controlled gas tube, a cold cathode controlled gastube. The particular advantage to be gained is that the need forfilament power for the control tube is eliminated, and furthermore,since many hot cathode controlled gas tubes have cathodes which requirea longer heating time than the oathodes of the tubes used in theassociated channel amplifiers, there of course is some delay between thetimes when the two types of tubes are conditioned for use after theheating circuit for the cathodes has been closed; Since controlled gastubes must have the anode potential interrupted or reduced in order torestore the control of the control electrode, various methods anddevices must be provided. In certain instances it is possible to provideimproved arrangements winch, for certain situations, may have distinctadvan tages over arrangements such as those already known in the art.For example, where arr-alternating current supply is used to bring aboutthe periodic interruptionof the conductivity of the control tube, thereis a possibility of some delay occurring in accordance with theprobabilities relative to the positive half-cycles of alternatingcurrent. In other words',-in certain instances the initiation-0fspeech'might fall within the negative loop of alternating current whichwould provide a maximum delay not less than 8.35 milliseconds or of asecond, which, together with the two or three milliseconds required fora positive half-cycle of anode potential to reach a value at which thetube will become conductive, might produce at certaintimes some speechclipping. In order to reduce such possibilities of speech clipping ordelay which might "be unde sirable for certain purposes, it is proposedto provide a direct current potential for thfi control tube which,together with some mean responsive to the conductivity of the tube Willperiodically of proper polarity the particular arrangement of the renderthe tube non-conductive. Such periodic rendition of the tube tonon-conductive state is I preferably produced at a frequency which islow compared to production voice frequencies, but which, however, issufficiently high so that it will not be difficult to provide a filterarrangement adequate to remove the noise which is brought about as aform of an alternating current component due to interruptions of theconductivity of the tube. If such an arrangement has providedperiodically interrupted conductivity of the tube by a variation of theanode voltage, the conductivity is re-established at the next half-Wavefrom the controlling voice or signal Wave. It therefore becomes apparentthat it is desirable to avoid a relatively high frequency ofinterruption because this would result in a smaller proportion of timeof conductivity of the control tube which, in turn, would produce asmaller average anode current flow and a resultant smaller controlvoltage for application to the associated channel amplifier.

In accordance with the present invention several embodiments aredisclosed for periodically rendering the control tube non-conductive. Inone of these embodiments circuit means are included in the anode circuitof the control tube which, due to the flow of anode current built up anIR drop which, in turn, suificiently reduces the anode potential so asto bring about themterruption of the conductivity of the tube. In theother embodiment other means are provided responsive to the conductivityof the tube for periodically rendering the tube non-conductive, and inthis arrangement a predetermined control voltage is constantly appliedto increase the gain of the associated channel amplifier and yet theanode supply to the controlled gas tube is periodically interrupted soas to periodically give the control electrode a renewed opportunity tocontrol the conductivity of the tube. Upon cessation of voice or signalcurrent flowing through the associated channel amplifier the failure ofthe control electrode to supply a potential sufiicient to renew theconductivity of the control tube brings about operation of aslow-to-release relay which thereby removes from the control electrodesof the tubes of the channel amplifier the fixed bias which was appliedto increase the gain thereof.

It is an object of the present invention to provide improved telephonesubstation apparatus of the character described wherein the channels arepreferably only partially blocked when not in use, the blocking of thesignal current transmitting and receiving channels is accomplished in asimple and reliable manner, and wherein the control circuits arearranged to effect channel blocking and unblocking operations withoutany substantial speech clipping. 7

It is another object of the invention to provide an improvedtransmission system particularly suited for use in telephone substationcircuits of the character described, wherein the control or channelblocking circuit is arranged in an improved manner to utilize a coldcathode controlled gas tube. a

It is still another object of the invention to provide an improvedtransmission system utilizing channel amplifiers wherein the controlcircuit employs a controlled cold cathode gas tube and wherein means areprovided responsive to the conductivity of the tube for periodicallyrendering the tube non-conductive.

Further features of the invention pertain to circuit elements, wherebythe above and additional operating features are obtained.

The novel features believed to be characteristic of the invention areset forth with particularity in the appended claims. The invention, bothas to its organization and method of operation, together with furtherobjects and advantages there- 'of-will bejst'be understood by referenceto the following description taken in connection with theaccompanyingdrawings, in which Figure 1 illustrates a substation circuit havingincorporated therein one embodiment providing certain features of theinvention as briefly outlined above; and Fig. 2 illustrates a differentembodiment of the improved transmission system as incorporated in thesubstation circuit.

Referring now more particularly to Fig. 1 of the drawings, thesubstation circuit therein illustrated is adapted to be connected to atelephone line I00 which may, for example, form a part of a conventionalautomatic or manual telephonesystem, in which case the distant endthereof willlterminate in a line circ t, the character of which isdetermined by the character of the exchange. Briefly described, thesubstation circuit comprises a transmitter or microphone ItII, receivingmeans in the form of a loudspeaker I02,

an anti-sidetone impedance network' or hybrid.

system I33, a. balancing circuit therefor I04, and outgoing and incomingamplifier channels I05 and I96. The outgoing amplifier channel maycomprise a plurality of stages of amplification of any suitable type,such as resistive, capacitive or inductive coupled amplifiers. Since theparticular type used is of no special significance to the presentinvention. only the input'and output tubes thereof have been disclosedto show how the control circuit associated therewith is to be connectedto the amplifier channel. The outgoing amplifier channel is shown havingan input vacuum tube Iil'l which has a grid energized from voicecurrents produced by the microphone "II. The grid to cathode circuitincludes a self-biasing resistor IDBsuitably by-passed by a capacitor109, both of which are connected to ground. A portion of the'outputstage of the amplifier is shown comprising the output tube I2I having agrid connected to a coupling capacitor I22 and a coupling resistor I23.The anode of the vacuum tube I2I is connected to'a transformer I24,

the output of which is connected to the hybrid be amplified by asuitable amplifier I29 which is provided with an output transformer I30.The controlcircuit I25 includes a cold cathode gas tube I3I having acathode, an anode, and a con trol or starting electrode. The control orstarting electrode of the gas tube 'I3I is connected to a predeterminedpointon a bleeder circuit comprising the grounded output winding of thetransformer I3U'and' the resistors I32 and I33 which are connected'tothe positive terminal of a source of voltage I34. The control electrodeis connected to the common juncture of the resistors I32 andI33; The

7 source of potentiall34 has its negative terminal connected through 'a5 resistor I35 to "ground. The cathode of the control"tube*'I3I isconnected through a resistor I36 to ground. The anode of the controltube I3I is arranged'to be connected through a suitable circ'uit to thepositive terminal of the source of voltage "F34. As has been stated, thecontrol electrode "of the tube I3] is connected to a bleedercircuit-which is arranged so that the voltage drop appearing across theresistor I32 supplies the greater portion of the voltage required by thecontrol electrode to render the control tube conductive. The remainderof the necessary required voltage is obtained from the voice components'f 'ppearingacross the output winding of the transformer I30. While thecontrol electrode of the vacuum tube I31 might be energized solely fromvoice components appearing across the transformer 1-30 by supplyingsufficient stages of amplification in the amplifier I29, it is believedto be an uneconomical arrangement. When anode current flows it will benoted that the anode to cathode circuit includes both the resistors I35and I36, the common juncture of which is connected to ground. Theopposite terminals of the resistors I'35and I36 are arranged to supplysuitable-potentials to the incoming channel amplifier thereby to disablethat channel amplifier, and tofthe outgoing channel amplifier thereby toenable that amplifier. Accordingly, the remaining terminal of theresistor I36 is connected to a suitable circuit including a resistor I37and a grounded capacitor I38 and the grid coupling resistor I 23 of theoutgoing channel amplifier outputtube 'I2I. The other terminal of theresistor I35 'isconnected to a circuit including a series resistor I39and a grounded capacitor I40 and the grid circuit of the inputtube ofthe incoming amplifier channel I06.

As has previously been stated, it is desired to provide "a controlcircuit wherein, in response to the conductivity of the tube, means areoperative for periodically rendering the tube non-conductive One of thesimplest circuit arrangements which would thus provide such operationwould be to "provide a resistor I44 connected in series between theanode of the control tube I 3| and thepositive potential of theanodesupply I 34-by connecting the strap I45 and disconnecting thestraps 146 and 141. By proper selection of the valueof the resistor I44in series with the anode to cathode circuit,'the resulting IR dropproduced inresp'onse to the conductivity of the tube I3I will be such asto reduce sufiiciently the efiective anode voltage that the tube will nolonger remain conductive. .Thereupon the voltage appearing across the.series resistor I44 also disappears and again the'full anode potentialis applied to the control tube I3I. If it is assumed that signalcurrents arestill being transmitted through the'outgoin channelamplifier the next succeeding positive half-wave of voice voltageappearing at the controlelectrode of the control tube I 3I will againrender the control tube conductive. This cycle of operation thereforewill continue to recur as long as voice currents are being transmittedthrough-the outgoing amplifier channel. Such an arrangement, however,would require that considerable filtering action be provided for theresistors I35 and I 36 in order to suppress the rather low interruptionfrequency resulting from such operation, which of course would reduceseriously the-rapidity with which the enabling and disabling unit I25could enable and disable the outgoing. and incoming channel amplifiersI35 and I06. 'In 'order to obviate this disadvantage another arrangementhas been provided, whereby "the initial enabling and disablingpotentials supplied to thechannel amplifiers have -a relatively small"amount of filtering action provided and subsequently additional filtercircuit elements are introduced in order to filter out the eife'cts ofstarting and stopping the control tube I3'I.

This improvement which provides for more rapid enabling and disablingaction is accom'- plished by disconnecting the strap I45 andconnectingthe stra'ps I46 and I4! of a circuit which may be'traced fromthe anode of the control tube I3I,strap I46, a'relay I it connected inseries with a 'resi'stor I48 which, together with the relay H0, isbyepassed by a capacitor I49 through the strap 1'4! to the positiveside-of the anode potential I34. A voltage-divider circuit comprisingthe resistors and I5:I is connected across the source of potential I 34.The common juncture between the resistors I5ll-and I-5I'is connected toground. The relay-III) is provided with break contacts I I I connectedbetween a resistor I52 which is connected to-anad-justable contact onthe resistor I50, and a'filter capacitor I53 which is connected toground. Make contacts IIZ are provided for connecting the capacitor I53in parallel with a small filter capacitor Ml] which is connected to oneterminal of a resistor I39. Another pair of contacts are also providedfor the relay IN] which include the break contacts II3 connected betweena resistor I54 which is connected to the adjustable contact of resistorI 5I, and a filter capacitor I55. By means of make contacts II4 thefilter capacitor I55is arranged to be connected in parallelto the,

smallercapacitor I353 which is connected to one terminal of resistorI31.

In thecircuit just described, initial energizati-on of the control tube-I3I produces a fiow oi anode current which rapidly produces potentialsacross theresistors I35 and I36 so that these potentials-are impressedthrough the resistors I 39 and I31- respectively upon the incoming andoutgoingchannel amplifiers. Thereupon the-relay I Illattractsitsarmature thereby to break con tacts-J-II and I-I3'and to make contactsH2 and II4. Thus shortly after the initial enabling anddisabling-voltages are supplied to the channel amplifiers, additionalfiltering action is supplied by connecting in circuit the capacitors I53and I555. The relay I I0 is provided with the capacitor I 49 so astoprovidea holdover action, so that the relay does not respond to-theperiodic and momentary interruptions of the control tube I3I. The anodecircuit of the control tube I3I is therefore provided with a relay I I 0and with a bridgingcapacitor I49 which offers the required impedance,together with the resistor I48 which is suflicient to periodicallyrender non-conductive the control tube I3 I due to the IZ drop. In thisnew arrangement therefore the interruption frequency is dependent uponthe supply voltage I34 and the total'ei fect of resistors I35, I36-andI48. In order that the connection of the'capacitors I53 and I55should-not disturb the value of the enabling and disabling voltagealready applied to the channel amplifiers, the capacitors I53 and I 55are initially charged through the break contacts II I and 'I I3ofthe-relay H0 and the current limiting resistors I 52 and I54from thevoltage divider resistors 15!) and I5I which are connected across thesourcerof potential I34.

The incoming channel amplifier I06 may comprise any desired number ofstages ofamplifica tiondependent upon the requirements to'be met, and-for purposes of simplicity only two amplifier tubesian input 7 tube I56and an output tube, I51, The control grid electrode of tube I56 isenergized from a have been shown.

the vacuum tube I56 may be biased negatively so as to disable theincoming channel amplifier whenever the outgoing channel amplifier isused,

and this is accomplished by connecting one terminal of the resistor I58to one terminal of a re- .sis'tor I39 of the enabling and disabling um'tI associated with the outgoing amplifier. The output tube I51 isconnected to an output transformer I59 which, in turn, is connected tothe loudspeaker I62. The grid circuit of th output tube I61 may includea resistor I60, one terminal of which is connected to an enabling anddisabling unit I6I associated with the incoming channel amplifier.

A portion of the incoming energy supplied across the input resistor I58of the incoming channel amplifier is coupled through a capacitor I62 anda voltage divider resistor I63 to a suitable amplifier I66 which isconnected to a coupling transformer I55. The secondary winding of thetransformer rounded, is connected through a resistor I66 to the controlelectrode of a controlled gas tube I61. This gas tube IE1 is of the sametype as the gas tube I3I of the other enabling and disabling unit I25.While in each of these cases the gas tubes represented have been shownas comprising an envelope containing a gas or other suitable ionizablemedium with an anode, a cold cathode, and a control electrode shown inthe form of a starting electrode, it of course is understood that thecontrol electrode may comprise any one of the other forms of controlelectrodes commonly used with this type of cold cathode gas tubeincluding a control grid. The control electrode of the gas tube I61 isconnected to the common juncture of the resistor I66 and a resistor I68I65, which has one terminal which, in turn, is connected to thepositiveterminal of a suitable source of potential I69 so that th circuitcomprising the transformer I65 and the series resistors I66 and 966constitute a bleeder circuit similar to the bleeder circuit provided forthe enabling and disabling unit I26. The cathode of the gas tube IE1 isconnected to the negative side of the source of voltage 566 through apair of resistors I10 and HI, the common juncture of Which is connectedto ground.

the secondary winding of posted to ground to and I83 which'are connectedto grou'nded'ca 'pacitors I18 and I16 and to current limiting'resistorsI85 and I86. The resistors I85 and I86, in turn, are connected tomovable contacts on a voltage divider comprising a pair of resistors I81and I86 connected across a source of voltage I69. The common juncture ofthe resistors 'I81and I88 is connected to ground. The cathode of the gastube W1 is connected eluding a resistor I89 to one terminal of the gridcoupling resistor I60 of the output vacuum'tube 61; The juncture betweenthe resistors I89 and I6!) is provided with a capacitor I96 which iscon-'- filter'action. The negative terminal of the battery I69 and oneterminal of the resistor 'I1I are connected by a circuit having a seriesresistor I9I which is provided with a grounded filter capacitor I92, tothe grid circuit of the input tube I61 of the outgoing channelamplifier.

,In response to signal energy appearing across the input of theincomingchannel amplifier the control electrode of the gas tube I61 hasits potential increased sufficiently so as to'ionize the tube, therebyto render it conductive. Thus anode current will flow through the,relayIBll and therresistor I16 so that shortly thereaiter the IZ (honorthose two elements will be sufiicient to reduce the anode voltage belowthat re-. quired to maintain conductivity of the gas tube I61. When thisoccurs control is again restored to the control electrode of the gastube. If signal energy is still being impressed on the in-,

coming amplifier the gas tube I61 will again be' rendered conductiveupon the next positive halfcycle of voice or signal energy. The relayI86, when energized, will disconnect the capacitors I18 and I19 from theresistors I85, I81, I86 and I88 whereby they were being charged to apredetermined potential, and thesecapacitors will then be connected bythe make contacts I82 and I84 7 thereby to filter adequately, thedisabling voltage supplied to the outgoing channel amplifier and theenabling voltage supplied to the output tube of the incoming channelamplifier The capacitor I11 connected in parallel to the resistor I16and the relay I8!) provides sufiicient holdover or slow releasecharacteristics so that the additional filter capacitors I18 and I16 arenot disconnected at the time. that the anode voltage of the gas tube I61is reduced sufficiently If an desired to provide a simplified formertube, a series resistor I12 may be connected by completing the anodecircuit through the strap J13 and disconnecting the straps I14 and I15.Theoperation of this circuit would'be similar, therefore, to the circuitdescribed for the outgoing amplifier enabling and disabling unit I25whenthe resistor I44 was connected in the anode circuit of the gas tubeI3I. Q 1 e l t is preferable, however, to use a circuit which hascertain, advantages over this simple circuit and, hence, the strap I13is disconnected and the straps I14, and I15 are connected in order todisconnect the resistor I12 and to connect in the anode'circuit of thegas tube I61 a relay I86 and a resistorsI 56 connected in' series witheach other to restore control to the control electrode of the tube. If,however, no further incoming signal energy is present thegas tube I61will not be rendered conductive upon the next positive half-cycle ofsignal voltage which of course is no longer there, and hence the relayI66 will release. It is believedthat no further explanation of theoperation of. this enabling and dis' abling unit ISI is necessary inView of the simie larity of circuit elements and operation to thecircuit elements and operation of the enabling and disabling unit'I25associated with the outgoing channel amplifier which previously wasdescribed in considerable detail;

From the above description it therefore becomes apparent that eachchannel amplifier has associated therewith an enabling anddisabling unitwhich includes a cold cathode controlled 'gas, tube whichjin response tosignaljcurren'ts traversing its associated channel amplifier, sup:

'pli'e's control potentials; one fof' which enables its channelamplifier and the other of which disables theother channel amplifier;Each of the control tubes is'connectd in a circuit which through acircuit in-' provide a small amount of is responsiveto the. conductivityof the tube to: periodically interrupt the. conductivity thereof so asto restore the; control of the. tube to the control electrode. In thepreferred circuit arrangement the initial enabling and disablingvoltages are provided with a relatively small amount of filter actionwhich has a relatively short time constant so that rapid enabling anddisabling may be accomplished, thereby to minimize orpreventspeechclipping. Subseq-uentto the initial application of therapid" enabling and disabling control voltages additional filter meansare connected so as to provide controlvoltages which have no appreciablealternating current components, and the additional filter means alsoserve to prevent feedback between the two channel amplifiers through thecontrolcircuits.

Referring to Fig. 2, another embodiment is shown wherein means areprovided for disabling and enabling incoming and outgoing channelamplifiers. In order to simplify the disclosure and the description,those elements which are similar to th elements: previously shown inFig. 1 have been given reference numerals one hundred greater than thenumerals in Fig. 1. The substation circuit there illustrated isconnected to a telephone line 200 which, in turn, is connected to ahybrid unit 203 which interconnects a balancing circuit 204, an outgoingchannel amplifier 205 and an incoming channel amplifier 295. Themicrophone is connected to the outgoing channel amplifier and aloudspeaker 282 is connected to the incoming channel amplifier. Theoutgoing channel amplifier is provided with In order that the enablingand disabling unit 225-may be responsive to signal: currents passingthrough the outgoing channel amplifier 205, a portion of the signalenergy is obtained by a voltage divider 226 and a coupling capacitor 221from the input circuit of the vacuum tube 201. This energy maybeamplifiedby a suitable amplifier 229 which is" connected to atransformer 230,, A cold cathode controlled gas tube 23| has a controlelectrode connected to av bleeder circuit comprising the groundedsecondary Winding of the transformer 230 and resistors 2.32 and 233which are connected to the positive side of a source of voltage 234. Thecathode of the gas tube 2M is connected to the negative side of thesource of voltage 234 through a pair of resistors 235 and 236, thecommon juncture of which is connected to ground. Before the gas tube 23|is rendered conductive there is. normally provided an anode circuitwhich includes a relay 2 l0, break contacts 2| a current limiting,resistor 248, and a choke coil 300' which is connected to the positiveside of a'source of voltage 234. The juncture between'the choke coil300- and the current limiting resistor 248' is connected to a groundedfilter capacitor 3M. The choke coil 300 and the capacitor 3M areprovided to filter out the transient impulses occurring from repeatedopening and closing of the contacts 2| I. The value of the resistanceand impedance of the choke coil 300', the relay- 2I||l and the resistor248 is such that-the: anode current is limited to a predeterminedmaximum amount, The juncture of the resistor 248 and the choke 300 isconnected to a resistor'392 which, in turn, is connected to a resistor333, one terminal of which is grounded. The juncture. oftheresistors302' and 303.'i connected to the break'contact 2|'2 of the relay 2H);The break contact 2|2 is connected to a capacitor 394 which, in turn, isconnected'to the negative terminal of the voltage source 234. Thecapacitor 304 is of such size as to provide sufii cient h-old'overcharacteristics to the voltages. developed across the. circuitcomprising the resistors-235 and 236: inparallel with the capacitor 3G4,so that the control voltages supplied to the incoming and outgoingchannel amplifiers hold over word and syllable pauses. The capacitor334, by virtue of its connection to the juncture between. the resistors332 and 303, is ini ially con nected to a bleeder circuit which chargesthe capacitor so that when it is connected by the make. contacts 2|3 toone terminal of the resister 236 there Wilt not be any anp eciabe cha einthe voltages appearing across the resistors 235 and 23.6 which areabpled to the cha nel amplifiers. One terminal of the resistor 236 isconnected throu h a resis or 231 to the grid coup ing resistor 223. ofthe output vacuum tube 22! of the outgoing channel amplifier. nal' ofthe resistor 231' is provided with a capacitor 238 conne ted to ground.One terminal of theres stor 235 is connected through a resistor 239which is provided With a grounded capacitor Miland thiscircuit isconnect d to the input resistor 258 of the i ut tube 256 of the incomingchannel amplifier 286.

The reay 2 ll! is so arran ed that upon the con ductiv ty of the gastube 23| the first operations that will occur w ll be the interruptionof the break contacts 212' and the closing of the make contacts 2| 3;During the initial flow of current from the anode to the cathode the vltage develooed' across the resistors 235 and 236 is suppl ed to thecontrol circuits of the vacuum tubes 255 and 22f of' the incoming andoutgoing channel amp ifiers, respectively, thereby to disable rapidlythe incoming channel amplifier and to enable rapidly the outgoingchannel amplifier. Subsequentto the initial application of the rapidenabling and disabling voltages the capacitor 334 is connected acrossthe resistors'235 and 236 to prov de a certain amount of smoothin andfiltering action which also serves to stabilize the control voltages sothat they will be effective to hold over word andsyllable pauses. As therelay 21m continues to attract its armature, eventually the contacts 2|I will be interrupted. The relay 2|0, having interrupted the anodevoltage, restores control of the tube 23| to the control electrode. Ifsignal currents are still passing through the outgoing channel amplifierthe tube 23! Will again become conductive, thereby a ain attracting thevarmature of the relay am. The contacts 2H are so arrangedthat making andbreaking of these contacts may occur as long as the speech continues' obe transmitted by the outgoing channel amplifier, make contacts 2 2 and2| 3'. Therefore While signal' currents are being t ansmitted throughthe outgoing channel amplifier, the relay 210 is in-' termittentlyenergized and deenergized so as to make and break the contacts 2 at arelatively rapid rate, thereby periodically restoring the control of thetube 23| to the control electrode. This operation continues until thereis a cessation of the-voice or signal currents through the outgoingchannel amplifier, whereupon the relay 2|ll is rel-easedso that thecontacts 2 are made and the contacts 2|2 and 2|3 are restored to thepositions shown in the figure. Therefore, in the arrangement-abovedescribed means are provided One termiwithout actuating the break orWhile a' single relay Zillhas responsive tothe conductivity of the coldcathode formthe operations of theanode voltage and 304 across theresistors 235 and 236, it of course will become apparent that theseoperations could be performed by several relays, or a relay and avibrator.

The incoming channel amplifier 20B is provided with an enabling anddisabling unit 26! which is similar in circuitarrangement and operationto the control unit 225 associated with the outgoing channel amplifier.nal energy appearing across the input resistor 258 of the incomingchannel amplifier is obtained by a' circuit comprisin a couplingcapacitor 262 and a resistor 263 which is provided with an adjustablecontact connected to a suitable ampli- The amplifier 264 is connected toa transformer 255, the secondary winding of which is grounded andconnected in a bleeder circuit including a pair of resistors 266 and 268connected to the positive terminal of the source of voltage 269. Thecontrol electrode of a controlled cold cathode gas tube 261 is connectedto the juncture between resistors 265 and 268. The anode circuit of thegas tube 261 includes a relay 280, break contacts ZBI of the relay 280,a current limiting resistor 216, and a choke coil 40!! which isconnected to the positive terminal of the source'of voltage 269. Asuitable grounded filter capacitor 40! is connected to the juncture ofthe resistor 216 and the choke coil 400. A bleeder circuit comprisingresistors 402 and403 connected between ground and one terminal of thechoke coil 40!] is arranged for charging a capacitor 484 through thebreak contacts 282 of the relay 280 to'a potential which approximatesthe average potential appearing across the resistors 210 and 2H. -Oneterminal of the resistor Z'IlLis connected through a resistor 289 to thegrid coupling resistor 26!] of the output tube 251 of the incomingchannel amplifier 206. The terminal of the resistor 289 which isconnected to the resistor 260 is also connected to a capacitor 290 whichis grounded so as to provide a small amount of filter action. When thecharged capacitor 404 is connected by the make contacts of relay-280 tothe juncture of the resistor 289, the resistor 21!) and the cathode ofthe gas tube 261, additional filtering action is obtained, One terminalof the resistor 2' is connected through a resistor 29! to the gridcircuit input tube 201 of the out oing channel amplifier. A suitablecapacitor 292 is connected between ground and one terminal of resistor29I to provide a certain amount of filter action.

The control circuit or enabling and disabling unit ZEI' operates inresponse to signal currents appearing across the input of the incomingchannel amplifier in a manner identical to the response of the othercontrol unit 225 so as to enable its channel amplifier and to channelamplifier.

While two'different arrangementshave been shown for controlling the gainof a pair of chafinel amplifiers operating in different directions, itwill be appreciated that in each instance the control circuit utilizes acold cathode controlled gas'tube and that means are provided in theanode circuit thereof which are responsive to the A certain amount ofthe sigdisable the other 1'2 conductivity of the control tube toperiodically render the tube non-conductive so as to periodically returnthe control of the tube to the control electrode. As long as signalcurrents continue to traverse the channel particular control tube, thecontrol tube will again be rendered conductive. The anode current whichflows produces control potentials across'a pairof resistors, and

a valuesufiicient to hold over the enabling and disabling action throughthe intervals during which the control tube is periodically renderednon-conductive.

While two embodiments been disclosed, it will be understood that variousmodifications may be made therein which are within the true spirit andscope of-the invention.

What is claimed is: V

1. The combination of and a control circuit therefor, said controlcircuit including a controlled gas tube having a'control'electrode, asource of anode potential con nected to said tube, means responsive tosignal currents passing through said amplifier for impressing a controlpotential on trode thereby to render said means connected in theanode-cathode circuit of said tube and having a relatively short timeconstant for generating a voltage to be applied to said amplifier tocontrol the gain thereof, de-

lay means, means operated by said tube for connecting to saidanode-cathode circuit said delay means to delay the time constant ofsaid voltage generating means; and means connected in said anode-cathodecircuit so as to be responsive to the conductivity of said tube forperiodically rendering said tube non-conductive.

2. The combination of a channel amplifier and a control circuittherefor, said control circuit including a controlled gas tube having acontrol electrode, a source of anode potential connected to said tube,means responsive to signal currents passing through said amplifier forimpressing a control potential on said control electrode thereby toinitiate. conductivity of said tube, means anode circuit of said tubeand responsive to theconductivity thereof a for periodically renderingthe said tube non-conductive.

3. The combination of a channel amplifier and a control circuittherefor, said control circuit including a controlled gas tube having acontrol electrode, a source of anode potential connected to said tube,means responsive to signal currents passing through said amplifier forcausing said control electrode to initiate conductivity of said tube,means connected in the anode circuit of said tube including aninterrupter responsive to the conductivity of said tube for periodicallydisconnecting said source of anode potential during the period of timethe signal currents pass through said amplifier, and means responsive tothe average anode current flow for applying abias to said amplifier tocontrol the gain thereof.

4. The combination of a channel amplifier and amplifier associated witha:

the potential thus obtained is maintained by suitable capacitors at ofthe invention have a channel amplifierthe control elec-' tubeconductive,

13; az-control: circuit thereto-r, said control: circuit in-- cludingaccntrolled gas'tub-e, a source of potena tial for said tube, meansresponsive to=signa1 currents passing through said amplifier for causingsaid tube tobecome conductive, meanshaving a relatively short timeconstant associated with said tube for deriving control voltagestherefrom to con-trol the gain of said amplifier, means i'n-l cludin acircuit interrupter connected in series Withsaid tube and said source,of potential, said interrupter being arranged in response totheconductivity of said tubetodisconnectsaid sourceof potential fromsaid tube, and means to be connected subsequently by said circuitwinter-- rupter in circuit with said. means for deriving control voltagestherefrom to increase the time constant thereof sufiiciently to supplyto. said amplifier a relatively stable potential to hold over Word andsyllable pauses in the signal currents passing through said amplifier,

5. The combination of a channel amplifier and a control circuittherefor, said control circuit including a controlled gas tube, a sourceof potential for said tube, means responsive to signal currents passingthrough said amplifier for causing said tube to become conductive, meansincluding an interrupter connected between said source of potential andsaid tube for periodically disconnecting said source of potential fromsaid tube in response to the conductivity" of said tube, meansassociated with said tube forderiving'control Voltages therefrom forcontrolling the gain of said amplifier, switching means controlled bysaid tube, and means arranged to be connected by said switching means incircuit with said means for derivin control voltages from said tube toincrease the time constant thereof sulficiently to supply a controlpotential proportional to the average current transmitted by said gastube. A

6. A transmission system comprising a pair of channels for transmittingsignal currents in different directions, each channel being providedwith a vacuum tube amplifier, a control circuit for each amplifierincluding a controlled gas tube arranged to be rendered conductive inresponse to signal currents passing through said amplifier, meansassociated with each gas tube for deriving control voltages therefrom,means for applying said control voltages to increase the gain of theassociated amplifier and to decrease the gain of the other amplifier,means actuated by said gas tube for subsequently connecting to saidmeans for deriving said control voltages means to increase the timeconstant thereof, and means responsive to the conductivity of said tubefor periodically rendering said tube non-conductive,

7. A transmission system comprising a pair of channels for transmittingsignal currents in different directions, each channel being providedWith a vacuum tube amplifier, a control circuit for each amplifierincluding a controlled gas tube arranged to be rendered conductive inresponse to signal currents passing through its associated amplifier,relatively lightly filtered means having a relatively short timeconstant associated with each gas tube for deriving control voltagestherefrom, means for applying said control voltages to increase rapidlythe-gain of the associated amplifier and to decrease rapidly the gain ofthe other amplifier, means for supplying additional filter action, meansactuated by said gas tube for subsequently connecting to said means forderiving control voltages said means for supplying additional filteraction, and means responsive to 14 the conductivity ofrsaid' tube forperiodically rendering said:tubenon-conductive.

8. A transmission system comprising azpairi of,"

channels for transmitting, signal currents in dif ierent. directions,each channel beingprovided with. a Vacuum tube amplifier, a controlcircuit, for: each amplifier including a controlled gas tube arranged toberendered conductive in response to; signal-currents passing throughits. associated: amplifier, a source of potential for said tube,meansassociated with each gas tube for deriving cmtrol voltagestherefrom, means for applying; said control voltages, to, increase thegain of. the: associated amplifier and to decrease the gain, of; theother amplifier, means connected in circuit with said gas tubepotentialapplied thereto sufficiently to interrupt; the. conductivity ofsaid tube, means for supplying; filter-action, and means operated bysaid tube for connecting, said filter means in circuit with said: meansfor derivin control voltages to increasethe time constant thereofsufficiently to,

hold, over the derived voltages for the time; of the periodicinterruptions of the conductivity of said tube;

9, A transmission system comprising a pair; of channels for transmittingsignal currents in dlf ferent directions, each channel being providedwith a vacuum tube amplifier a control circuit for each amplifierincluding a controlled cold cathode. gas tube, means for rendering saidgas tube conductive in response to signal currents passing throughits,associated amplifier, a source of potential for said gas tube, meanshaving a relatively short time constant associated with each gas tubefor deriving control voltages therefrom, means for applying said controlvoltages to increase rapidly the gain of the associated amplifier and todecrease rapidly the gain of the other amplifier, means for increasingthe time constant of said means for deriving control voltages, means forconnecting said last-mentioned means to said means for deriving controlvoltages in response to the conductivity of said tube, and meansconnected in circuit with said tube for producing in response to theconductivity of said tube a potential drop sufiicient to render saidtube non-conductive.

10. A transmission system comprising a pair of channels for transmittingsignal currents in different directions, each channel being providedwith a vacuum tube amplifier, a control circuit for each amplifierincluding a controlled cold cathode gas tube arranged to be renderedconductive in response to signal currents passing through saidamplifier, a source of anode potential for said gas tube, meansconnected in the anode circuit of said tube and responsive to theconductivity of said tube for producing a potential drop sufiicient tointerrupt the conductivity of said tube, means having a relatively shorttime constant associated with each gas tube for deriving controlvoltages therefrom, means for applying said control voltages to increaserapidly the gain of the associated amplifier and to decrease rapidly thegain of the other amplifier, a relay connected in the anode circuit ofsaid gas tube, and an energy storage device arranged to be connected bysaid relay in circuit with said means for deriving control voltages fromsaid tube thereby to increase the time constant thereof sulficiently tohold over the average derived voltage for a time interval equal to orgreater than the time of interruption of the conductivity of said tube.

for periodically reducing the cathode gas tube arranged to be renderedconduc tive in response to signal currents passing through saidamplifier, a source of anode potential for said gas tube, means having arelatively short time constant associated with each gas tube for'deriving control voltages therefrom, means for applying said controlvoltages to increase rapidly the gain of the associated amplifier and todecrease rapidly the gain of the other amplifier, a relay connected inthe anode-circuit of said gas tubeand responsive to the conductivity ofsaid tube for interrupting the conductivity thereof each time it isrendered conductive, an energy storage device arranged to be connectedby said relay in circuit with said means for deriving control voltagesfrom said tube thereby to increase the time constant thereofsufi'iciently to hold over the average derived voltage for a time inter;val equal to or greater than the time ofinterruption of the conductivityof said tube, and means for initially chargingsaid energy storage deviceto a potential substantially equal to the average control voltagederived by the means therefor. V

12. A transmission system comprising a pair of channels for transmittingsignal currents in different directions, each channel being providedwith a vacuum tube amplifier, a control circuit 16, for each amplifierincluding a controlled cold cathode gas tube arranged to be renderedconductive in response to signal currents passing through its associatedamplifier, a source of potential for each tube, means having arelatively short time constant associated with each gasv tube forderiving control voltages therefrom, means for applying said controlvoltages to increase rapidly the gain of the associated amplifier and todecrease rapidly the gain of the ther amplifier, means including acircuit interrupter con nected in the circuit supplying potential tosaid tube for periodically disconnecting from said tube said source ofpotential, and means arranged to' be connected by said interrupter incircuit with said means for deriving control voltages from said tube toincrease the time constant thereof suffi-. ciently to supply an averagecontrol voltage to said amplifiers as long as signal currents arepassing through the channel associated 'With said control tube. I

ROSWELL H. HERRICK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

